Mercury (Hg) is a highly toxic compound and exposure at appreciable levels can lead to adverse health effects for people of all ages, including harm to the brain, heart, kidneys, lungs, and immune system. Mercury is naturally occurring but is also emitted from various human activities, such as burning fossil fuels and other industrial processes. For example, in the United States about 40% of the mercury introduced into the environment comes from coal-fired power plants.
In the United States and Canada, federal and state/provincial regulations have been implemented or are being considered to reduce mercury emissions, particularly from coal-fired power plants, steel mills, cement kilns, waste incinerators and boilers, industrial coal-fired boilers, and other coal combusting facilities. For example, the United States Environmental Protection Agency (U.S. EPA) has promulgated Mercury Air Toxics Standards (MATS) which would, among other things, require coal-fired power plants to capture approximately 90% of their mercury emissions beginning in 2015.
The leading technology for mercury control from coal-fired power plants is activated carbon injection (ACI). ACI involves the injection of sorbents, particularly powdered activated carbon (PAC), into flue gas emitted by the boiler of a power plant. PAC is a porous carbonaceous material having a high surface area, which exposes significant amounts of beneficial chemically functional and catalytic reaction sites, creating high adsorptive potential for many compounds, including capturing mercury from the flue gas.
ACI technology has shown the potential to control mercury emissions in most coal-fired power plants, even those plants that may achieve some mercury control through control devices that are primarily designed for the capture of other pollutants, such as wet or dry scrubbers used to capture sulfur dioxide (SO2) and other acid gases from the flue gas stream. Acid gases and acid gas precursors in the flue gas stream typically come from three primary sources. The first is the coal feedstock fed to the boiler. Certain types of coal inherently have high concentrations of sulfur, nitrogen, chlorine, or other compounds which can form acid gases in the flue gas. For example, coals such as Illinois Basin coal with high sulfur content (e.g., above about 0.5%) are becoming more common as a boiler feedstock for economic reasons, as high sulfur coals tend to be cheaper than low sulfur coals. A second source is the selective catalytic reduction (SCR) step for controlling emissions of NOx. An unintended consequence of this process is that SO2 in the flue gas can be oxidized to form SO3. A third source is that the power plant operator may inject SO3 into the flue gas stream to enhance the efficiency of the particulate removal devices, e.g., to avoid opacity issues and increase the effectiveness of an electrostatic precipitator (ESP) in removing particulates from the flue gas stream.
Flue-gas desulfurization (FGD) is a set of technologies used to remove sulfur dioxide from exhaust flue gases of coal-fired power plants. One of the common methods is to use a wet flue-gas desulfurization (wFGD) unit (i.e., a wet scrubber unit) that uses a slurry of a calcium-based or sodium-based alkaline compound, usually limestone or lime. Contacting the flue gas with the slurry scrubs (i.e., removes) contaminants from the flue gas, forming calcium sulfite or sodium sulfite. In FGD systems utilizing forced oxidation, the corresponding sulfites are converted to sulfate byproducts that are collected while the scrubbed flue gas is emitted. The wet scrubber units have also been shown to be co-beneficial with respect to the removal of mercury. The wet scrubber unit is often the last emission control equipment before the stack, so it is critical to ensure that mercury capture is achieved in or before the wet scrubber unit.
According to the U.S. EPA sixty-nine percent of coal-fired capacity will be wet scrubbed by 2025, indicating a huge need for emission control in the wet scrubber unit. Mercury control additives may also be utilized in wet scrubber units, and may include sulfur-based compounds that can tightly bond with mercury to form mercury sulfide. However, a large amount of the additive (e.g., 1:1 to mercury) is typically required to achieve a high removal rate of mercury, adding to the cost of emission control. Additionally, sulfur-based additives only work well for mercury capture in low oxidation-reduction potential (ORP) environments, or may even bring the ORP down, which can cause the acid gas capture in the wet scrubber unit to be less effective.
In addition to mercury emission, stringent regulations in regards to emission of other contaminants, such as selenium, arsenic, nitrates, and bromine, have been proposed. In a forced-oxidation wet scrubber unit, selenium exists as selenite (SeO32−) or selenate (SeO42−), depending on the ORP in the aqueous phase. Selenate is more difficult to capture than selenite, due to its higher water solubility. Halogens such as bromine, which may be in the form of a bromide compound such as calcium bromide, are often used to enhance mercury capture in the flue gas stream by adding the bromide compound to the combustible material (e.g., coal) or directly to the flue gas stream, resulting in bromine build-up in the aqueous sorption liquid of the wet scrubber unit. Therefore, it may be necessary to monitor bromine levels in the wet scrubber unit. Although bromine in oxidized forms, e.g., bromine (Br2), bromate (BrO32−), etc. can be easily adsorbed by PAC, in low ORP conditions, the bromine exists as bromide (Br−) and therefore may not be readily captured by PAC. The wet scrubber unit serves as the last opportunity to capture target emission compounds such as mercury, selenium, arsenic and bromine, as well as potentially arsenic and nitrates. Therefore, effective capture of these and other contaminants in the wet scrubber unit is crucial.